Metallized paper is typically produced by a process of direct high-vacuum metallization. In this process, the substrate that is going to be metallized is initially coated with a layer of film to produce a smooth surface suitable for metallization. The coated paper is introduced in a high vacuum chamber where a vaporized metal, normally aluminium, is applied on its surface. This metal surface is coated with another layer of film to protect said metal layer and prevent it from oxidizing. The layers applied comprise, in general, quantities between 1 and 3 g/m2.
The polymers used for these films or coatings in these applications are of a very varied nature, such as acrylic derivatives, styrenic derivatives, epoxy resins, nitrocellulose, etc. However, all the compositions used in this process have in common the capacity to form a film when the solvent used evaporates.
The layer of coating applied before the metallization should combine a series of requirements such as said surface smoothness, good adhesion of the metal and properties which give a metallic appearance to the paper once metallized. As regards the layer applied after metallization, it should have good surface coating to avoid oxidation of the metal, good adhesion thereto and also a series of requirements that depend on the final application of the finished product.
The application of these compositions on paper is normally performed using gravure techniques. This technique has the main drawbacks of low processing rates and narrow Theological margins which the compositions have to maintain to achieve good application, always within a low viscosity zone.
Following these manufacturing processes, the rates reached do not exceed 400 m/min. Despite constant technological advance, the printing processing rates using gravure are quite far from those produced by other methods typical in paper processes.
With respect to the rheological requirements of the compositions to apply using gravure, it is necessary that they maintain low levels of low-shear viscosity as, otherwise, neither a good transfer of the composition to the paper, nor good levelling, nor a good surface finish thereof, is achieved. This Theological requirement drastically limits the solids content in the composition used, typically not exceeding 30% in the case of compositions in solvent medium and 35% in the case of aqueous medium. In the case of compositions in aqueous medium, this relatively low solids content of the compositions hampers the development of formulations suitable for the process that replace the current compositions in solvent medium.
Another limitation derived from the gravure technique refers to the layer's finish once this has been formed: typically, the layers thus applied all have a series of small surface defects such as small holes, craters and microchannels. These discontinuities in the film thus formed significantly reduce its light, vapour, water and oxygen barrier properties.
In the state of the art, there, therefore, exists the need to provide a method to manufacture metallized paper which overcomes part or all of the limitations of the aforementioned processes of the state of the art.